The following abbreviations are used in the description below:
3GPP third generation partnership project
CQI channel quality information
DL downlink
e-NodeB Node B of an E-UTRAN system
eNBr relay-enhanced eNB
E-UTRAN evolved UTRAN
LTE long term evolution of 3GPP UTRAN (E-UTRAN or 3.9G)
Node B base station or similar network access node, including e-NodeB
RAN radio access network
RBR radio band resource
RN relay node
RS relay station
QoS Quality of Service
UE user equipment (e.g., mobile equipment/station)
UL uplink
UMTS universal mobile telecommunications system
UTRAN UMTS terrestrial radio access network
3GPP is standardizing the long-term evolution (LTE) of the UMTS radio-access technology which aims to achieve reduced latency, higher user data rates, improved system capacity and coverage, and reduced cost for the operator. Future LTEs standard releases (here termed Release 9 for brevity) may use relay nodes (RNs), alternatively termed relay stations (RSs), in order to enhance coverage areas in the cell of an e-NodeB. FIG. 1 gives an overview of how such RNs can be used to advantage: to extend wireless coverage to the interior of a building, to extend coverage beyond the cell edge (cell edge as defined by the radio range from the e-NodeB), to direct radio signals more particularly to a valley between buildings or to a radio-frequency ‘shadow’ behind a building, to extend the cell to other non-contiguous areas such as via multi-hops/multi-relays, and to provide robust radio signals in any other ‘coverage holes’ that may be at different areas of the cell. The RNs can be fixed or mobile, such as mounted to a high-speed train. In some systems the relay stations may be opportunistically available UEs/mobile terminals that are not owned by the network itself. For clarity, a network access node that employs RNs is termed a relay enhanced access node, or in the context of LTE it is termed an e-NBr for brevity.
Apart from this main goal of coverage extension, introducing relay concepts in LTE can also be used to aid in the provisioning of high-bit-rate coverage in high shadowing environment, to reduce the average radio-transmission power at the UE which extends the UE's battery life, to enhance cell capacity and effective throughput (e.g., by increasing cell-edge capacity and balancing cell load), and to enhance overall performance and deployment cost of the radio access network RAN.
After being carefully considered in pre-standardization activities like the Wireless World Initiative WWI/Wireless World Initiative New Radio WINNER system concept, relay systems are achieving the level of maturity that is needed in ongoing LTE standardization activities. The WINNER concept seeks to develop a system that is QoS aware and uses intelligent scheduling to meet user demands and physical constraints, in a model that is scalable for deployment to any of various spectrum types and bandwidths including spectrum sharing. This scalable deployment gives rise to relay nodes being an important part of the WINNER concept. As an example of later stages of relay node development, the IEEE 802.16j standardization adds relays to the IEEE 802.16e standard. This recent development has increased the pressure to consider relays also in LTE standardization. Certain wireless network operators have been actively pushing for relay standardization since it is expected that relay systems will be economically viable due to reduced backhaul and site acquisition costs. In order to keep LTE competitive it is more than probable that relay extensions to LTE are to be studied within 3GPP in the release 9. Various topics will be studied and relays appear likely to play an important role in LTE Release 9, and RSs are likely to be included in the LTE Release 9 agenda.
There are many specific types of relay systems proposed, from the simple (e.g., amplify/forward implementations, applied in single frequency networks such as digital video broadcast for handhelds DVB-H, for example) to the more complex such as using network coding to improve the overall performance. A common relay type that is proposed for cellular relaying is a detect/forward type of relay, where an input signal is detected and retransmitted using the same procedure as in the original transmission. The following discussion assumes the detect/forward type implementation for a relay network.
To ensure economic viability in adopting RNs into the LTE network, backward compatibility between Release 8 (standardization ongoing) and Release 9 will be needed. A reasonable assumption is that full backward compatibility is required from the UE perspective, i.e. Release 8 and Release 9 terminals should work equally well in Release 8 and in Release 9 networks. At the network side software and even hardware updates between standard releases may be possible but preferably they should be as small as possible. Hence, from the UE viewpoint the serving network node should function in exactly the same way as the e-NodeBs of Release 8. Due to this requirement, the reduction of functionalities of the e-NodeB when defining and implementing relay nodes will be difficult, and the relay nodes will need to support all of the main e-NodeB functions also. Due to this fact it can be assumed that relay nodes are capable of flexible resource sharing with the e-NodeB that controls them.
Assuming no RNs in an LTE (also known as E-UTRAN or 3.9G) cell so that the UEs and the e-NodeB communicate directly, the link adaptation and scheduling procedures utilize channel quality information (CQI) reports from the active UEs. An ideal CQI report tells the e-NodeB the quality of each radio band resource (RBR) that the corresponding UE being served measures. The e-NodeB can utilize this information for optimal scheduling and link adaptation. However, ideal CQI reporting on an RBR basis is not practical due to the limited number of pilot symbols available and also to the need to control the volume of control signaling overhead. The prior art does address several low bandwidth CQI schemes, the most representative ones seen to be offset CQI, threshold CQI and best-M CQI reporting.
The introduction of RNs can have an impact on the overall architecture of the network as well as the CQI reporting and its usage for scheduling and link adaptation. The scheduling of UEs under control of a RN can be done by the RN with the help of the controlling e-NodeB (e.g., where the RN is given some authority to schedule radio resources given by the e-NodeB), or solely by the controlling eNBr (where the RN acts as a communication conduit and all scheduling decisions are by the e-NodeB).
Certain problems arise in the former case. The RN may be doing the scheduling, but the impact of erratic radio channels has to be taken into account so that the controlling eNBr is able to assign sufficient radio resources for the RN to operate and provide adequate quality of service (QoS) to each relayed UE. For instance, a situation may arise when a certain relayed UE's buffer in the RN is being overfilled because the RN-UE link for that UE is very bad (low CQI) and it is not being scheduled by the RN, while at the same time the e-NodeB keeps sending data for that UE in the eNBr-RN link, which has a sufficiently good link i.e. high CQI. This is a flow control problem with no analogy to the case where the link is direct between the UE and the e-NodeB and there are no RNs between them.
The introduction of RNs is a new concept in LTE. Thus, scheduling and associated CQI reporting has not been considered previously. The case where RNs control the scheduling of its own users is similar to the non-transparent mode defined in WIMAX. However, the issues regarding CQI reporting and scheduling are not resolved since those are different in LTE.
What is needed in the art is an approach to optimize scheduling of UEs under control of a RN for various actual channel conditions that exist between the RN and the UEs under its control, preferably in a manner that is consistent with LTE.